1. Field of the Invention
This invention relates to a signal processing method and apparatus for detecting faults and analyzing components that rotate in rotating machinery, and, in particular, it relates to a method and apparatus which analyzes signals from shaft encoders to detect and isolate resonances of these components.
2. Description of Related Art
Machinery shafts and components that are fixed to shafts, such as rotor heads and turbine or compressor blades, may develop cracks or other physical changes during normal use. Such deterioration is, however, generally difficult to detect while the machinery is operating, or even when the machine is disassembled for visual inspection. Vibration measurements, in the x, y or z direction are often used to detect developing problems in rotating machinery. The following prior art techniques may be relevant to this invention: "Method and Apparatus for Monitoring the Shaft Vibration of a Rotary Machine," Inventors Kurihara, Nobuo et al., U.S. Pat. No. 4,426,641; Russian Patent Doc. #0666454; Angular motion sensor, model 361 A. PCB Piezoelectronics, Depew, N.Y.; Torsional Vibration Analysis. A. R. Crawford, Machine View Incorporated, 1994. Application paper. Knoxville, Tenn.; and, "Method and Apparatus for Detecting Geer Defects," Walter C. Hernandez, Edward A. Page and Kenneth A. Lefler, U.S. Pat. No. 4,931,949. Such translational measurements have proven, however, to be relatively insensitive indicators of deterioration in the structural integrity or performance of rotating shafts or the components attached to them, such as the rotors and blades mentioned above. While deterioration in such shaft-associated components produce little translational vibration, they have a measurable effect on the instantaneous rotational velocity of the shaft. This variation is rotational velocity is referred to as the rotational, or torsional, vibration of the shaft. In an attempt to analyze torsional vibration, investigators variously have attached accelerometers or strain gauges to the rotating components and communication is accommodated through slip rings or radio frequency methods. Velocity sensors may be attached to a shaft to provide instantaneous angular velocity. Others have analyzed shaft encoder signals, which yield one or more electronic pulses per shaft rotation. In this, the rate of the encoder pulse train is often converted to a proportional voltage, which is then subject to spectral analysis. Some investigators manually examine the time domain pulses directly for variation in RPM rate. To date, the various methods of torsional vibration analysis have proven inadequate to detect machinery component deterioration except when catastrophic failure is imminent.
The current methods for the analysis of encoders to detect torsional vibration has been hampered by the effects of RPM drift, FFT leakage, and Amplitude Modulation (AM) in addition to random noise. As a result, the predictive information implicit in torsional vibrations in the rotation of a shaft has not been adequately isolated and detected by prior art methods used to date. The present invention incorporates a method and system for managing the various sources of noise and other masking signal components in a manner that enables the detection and measurement of key aspects of the fine structure of the torsional vibration of rotating shafts and their attached components.